The present invention relates to non-evaporable getter alloys.
Particularly, the invention relates to non-evaporable getter alloys which provide for a high efficiency in the sorption of gases, particularly of nitrogen.
Non-evaporable getter alloys, also known as NEG alloys, can sorb hydrogen in a reversible way and gases such as oxygen, water, carbon oxides and, in the case of some alloys, nitrogen, irreversibly.
A first use of these alloys is vacuum maintenance. Vacuum maintenance is requested in the most various applications, for example in particles accelerators, in X-rays generator tubes, in flat displays of the field emission type and in thermally insulating evacuated interspaces, such as in thermal bottles (thermos), in Dewars or in the pipings for oil extraction and transportation.
The NEG alloys can also be used for removing the above mentioned gases when they are present in traces inside other gases, generally noble gases. An example is the use in lamps, particularly the fluorescent ones which are filled with noble gases at pressures of a few tens of millibars, wherein the NEG alloy has the purpose of removing traces of oxygen, water, hydrogen and other gases, thus maintaining the suitable atmosphere for the lamp functioning; another example of removal of traces of the cited gases from other gases is the purification of inert gases, in particular for applications in microelectronic industry.
Generally these alloys have zirconium and/or-titanium as main components and comprise one or more elements selected among transition metals or aluminum.
NEG alloys are the subject matter of several patents. U.S. Pat. No. 3,203,901 discloses Zrxe2x80x94Al alloys, and in particular the alloy having weight percent composition Zr 84%-Al 16%, manufactured and sold by the applicant under the name St 101; patent U.S. Pat. No. 4,071,335 discloses Zrxe2x80x94Ni alloys, and in particular the alloy having weight composition Zr 75.7%-Ni 24.3%, manufactured and sold by the applicant under the name St 199; U.S. Pat. No. 4,306,887 discloses Zrxe2x80x94Fe alloys, and particularly the alloy having weight percent composition Zr 76.6%-Fe 23.4%, produced and sold by the applicant under the name St 198; U.S. Pat. No. 4,312,669 discloses Zrxe2x80x94Vxe2x80x94Fe alloys, and in particular the alloy having weight composition Zr 70%-V 24.6%-Fe 5.4%, manufactured and sold by the applicant under the name St 707; U.S. Pat. No. 4,668,424 discloses zirconium-nickel-mischmetal alloys, with optional addition of one or more transition metals; U.S. Pat. No. 4,839,085 discloses Zrxe2x80x94Vxe2x80x94B alloys, wherein E is an element selected among iron, nickel, manganese and aluminum or a mixture thereof, U.S. Pat. No. 5,180,568 discloses intermetallic compounds Zr1Mxe2x80x21Mxe2x80x31, wherein Mxe2x80x3 and Mxe2x80x3, either alike or different, are selected among Cr, Mn, Fe, Co and Ni, and in particular the compound Zr1Mn1Fe1 manufactured and sold by the applicant under the name St 909; U.S. Pat. No. 5,961,750 discloses Zrxe2x80x94Coxe2x80x94A alloys, wherein A is an element selected among yttrium, lanthanum, Rare Earts or a mixture thereof, and particularly the alloy having weight composition Zr 80.8%-Co 14.2%-A 5%, produced and sold by the applicant under the name St 787; finally, getter alloys based on Zr and V for use in gas purifiers are described in various patent applications published in the name of the firm Japan Pionics, among which for example the applications Kokai 5-4809, 6-135707 and 7-242401.
NEG alloys have different properties according to their composition. For example, the alloy St 101 is, among those mentioned, the best one as long as hydrogen sorption is concerned, but requires, for working, an activation treatment at relatively high temperatures, of at least 700xc2x0 C.; the alloy St 198 has poor nitrogen sorption properties, therefore it is employed for the purification of this gas; the compounds described in U.S. Pat. No. 5,180,568 do not sorb hydrogen As a result of these behavior differences, the choice of the NEG alloy to be employed depends on the specific foreseen application. In particular, it may be stated that, among these, the most largely used is the one named St 707, as described in U.S. Pat. No. 4,312,669, thanks to its good sorption qualities, in particular for hydrogen, and to the relatively low activation temperature required by this NEG alloy.
Removal of atmospheric gases is important in some applications. This is for example the case of the thermal insulation, wherein the gases which remain in the evacuated interspace during the manufacture have to be removed: as a matter of fact, in order to maintain the production costs within acceptable limits, the pumping of the interspace which is carried out before the sealing thereof is generally interrupted after a fixed time, generally leaving a residual pressure, although limited, in the interspace itself. The sorption of the atmospheric gases is also requested in the currently studied application of the energy inertial accumulators, better known with the definition xe2x80x9cfly wheelsxe2x80x9d, which work on the principle of rotating an object of high mass at high speed in an evacuated chamber, vacuum is necessary in this application in order to prevent the rotating mass from losing energy because of the friction with the gases present in the chamber. In these applications, particularly important for the choice of the NEG alloys is the behavior towards nitrogen, both because this gas forms about 80% of the composition of the atmosphere, and because it is the one, among atmospheric gases (with the exception of the noble gases), which is removed by the NEG with the highest difficulty.
The industrial application which currently requires the highest efficiency of undesired gases removal is purification of gases for the semiconductor industry. As a matter of fact, it is known that impurities in the process gases can be incorporated into the layers which form the solid state devices, thus causing electronic defects in them and therefore production rejects. The degrees of purity which are presently requested by the semiconductor industry are of the order of the ppt (10xe2x88x9212 in atoms or molecules). Therefore, the availability of NEG alloys having very high efficiency of impurity sorption is necessary; as above noted, nitrogen is, among gases which represent the normal impurities in a process gas, the one which is removed with the highest difficulty from the NEG alloys.
Object of the present invention is therefore providing non-evaporable getter alloys having high gas sorption efficiency, particularly nitrogen.
This object is obtained according to the present invention by non-evaporable getter alloys comprising zirconium, vanadium, iron, manganese and at least one element selected among yttrium, lanthanum and Rare Earths, having a percent composition of the elements variable within the following ranges (in the rest of the text, all percentages and ratios are by weight, unless otherwise specified):
zirconium from 60 to 85%;
vanadium from 2 to 20%;
iron from 0.5 to 10%;
manganese from 2.5 to 30%; and
yttrium, lanthanum, Rare Earths or mixtures thereof from 1 to 6%.